The present invention relates to immersion heaters for heating liquid in a bath and, particularly, to electrical resistance heaters formed of a continuous, flexible cable. Flexible cable resistance heaters are particularly suitable for immersion in corrosive chemical baths since the exterior of the flexible cable may be jacketed with a suitable plastic material having satisfactory resistance to the corrosive nature of the chemical bath being heated. An example of a flexible cable resistance heater is that shown and described in U.S. Pat. No. 4,158,764 issued to Frank J. Yane and assigned to the assignee of the present invention.
It is known to provide such flexible cable heaters with an outer casing or jacket formed of polytetrafluoroethylene (PTFE) material which, although has satisfactory resistance to chemical attack by corrosive liquid media, has the disadvantage that when employed in a thin wall for the desired flexibility, the permeability of the PTFE material has been found to permit transmigration of heated chemical vapor into the interior of the cable heater. It has been found in service that the accumulation of corrosive chemical vapor within the heater cable corrosively attacks the material of the coil heating element and causes early deterioration of the heating element and consequent failure of the cable heater.
Thus, it has long been desired to find a way for means of protecting a plastic jacketed, flexible cable immersion heater from the effects of accumulated permeation of hot chemical vapors into the interior of the cable heater and, yet, retain the flexibility and desirable heat transfer properties of the thin wall plastic jacket for the cable heater. It has further been desired to find a technique for preventing corrosive attack on the resistive heating element in a flexible cable heater without increasing the thickness or decreasing the heat transfer capabilities of the outer jacket of the cable heater.